Mechanisms used to examine the surface of a semiconductor wafer commonly use a wafer support known as a handling chuck. Modern semiconductor wafer inspection techniques may entail incrementally inspecting the surface of the wafer with a probe that moves laterally as the wafer rotates, much like a phonograph needle passes over a phonograph record. As may be appreciated, the greater the speed of rotation, the larger the number of sample wafers which may be examined for a given time.
Conventionally the wafer is mounted on an edge handling chuck and supported at three or four points at the edge of the wafer, e.g., using an edge clamp ring that is secured to the chuck. Air passes through a center hole in the chuck and is dispersed to the atmosphere using multiple pressure relief openings in the chuck near the edge of the wafer. The air pressure supports the wafer against the tendency to sag. The vacuum chuck is mounted on a spindle to permit rotation of the chuck and the wafer. During high-speed rotation, the gap between the wafer surface and the probe may vary due to wobble of spindle and chuck non-flatness. Such gap variation may be tolerable for a low-resolution scan, where the depth of focus is large. However, as the spot size of the “probe” reduces due to the requirement of improved sensitivity, the depth of focus reduces. For probes having a very small depth of focus, it is often necessary to adjust the focus for high-speed variations in the wafer height due to non-flatness and high speed of rotation of the chuck. Auto focus systems typically have limited bandwidths, which make it difficult to track wafer height variations at high rotational speeds. In addition, contact between the back surface of the wafer and the chuck surface may generate numerous particles, which can contaminate the wafer above an acceptable level and reduce the yield of usable devices from the wafer.
U.S. Pat. No. 5,511,005 describes a system for wafer processing including wafer measurement and characterization having vertical processing apparatus in which only the edge of the wafer is contacted as the wafer is rotated in a vertical plane. Since the wafer is held vertical as it rotates, particulate contamination and distortion of the wafer may be reduced due to the effects of the gravity. However, most current processing tools for measurement and characterization of semiconductor wafer are set up to measure or process a wafer oriented in a horizontal plane. Thus, vertical wafer rotation would probably require substantial and expensive reconfiguration, re-engineering and redesign and perhaps replacement of semiconductor wafer tools.
It is within this context that embodiments of the present invention arise.